A local translation program regulates centriole amplification in the airway epithelium

Biogenesis of organelles requires targeting of a subset of proteins to specific subcellular domains by signal peptides or mechanisms controlling mRNA localization and local translation. How local distribution and translation of specific mRNAs for organelle biogenesis is achieved remains elusive and likely to be dependent on the cellular context. Here we identify Trinucleotide repeat containing-6a (Tnrc6a), a component of the miRNA pathway, distinctively localized to apical granules of differentiating airway multiciliated cells (MCCs) adjacent to centrioles. In spite of being enriched in TNRC6A and the miRNA-binding protein AGO2, they lack enzymes for mRNA degradation. Instead, we found these apical granules enriched in components of the mRNA translation machinery and newly synthesized proteins suggesting that they are specific hubs for target mRNA localization and local translation in MCCs. Consistent with this, Tnrc6a loss of function prevented formation of these granules and led to a broad reduction, rather than stabilization of miRNA targets. These included downregulation of key genes involved in ciliogenesis and was associated with defective multicilia formation both in vivo and in primary airway epithelial cultures. Similar analysis of Tnrc6a disruption in yolk sac showed stabilization of miRNA targets, highlighting the potential diversity of these mechanisms across organs.

Biogenesis of organelles requires targeting of a subset of proteins to specific subcellular domains by signal peptides or mechanisms controlling mRNA localization and local translation. How local distribution and translation of specific mRNAs for organelle biogenesis is achieved remains elusive and likely to be dependent on the cellular context. Here we identify Trinucleotide repeat containing-6a (Tnrc6a), a component of the miRNA pathway, distinctively localized to apical granules of differentiating airway multiciliated cells (MCCs) adjacent to centrioles. In spite of being enriched in TNRC6A and the miRNA-binding protein AGO2, they lack enzymes for mRNA degradation. Instead, we found these apical granules enriched in components of the mRNA translation machinery and newly synthesized proteins suggesting that they are specific hubs for target mRNA localization and local translation in MCCs. Consistent with this, Tnrc6a loss of function prevented formation of these granules and led to a broad reduction, rather than stabilization of miRNA targets. These included downregulation of key genes involved in ciliogenesis and was associated with defective multicilia formation both in vivo and in primary airway epithelial cultures. Similar analysis of Tnrc6a disruption in yolk sac showed stabilization of miRNA targets, highlighting the potential diversity of these mechanisms across organs.
Biogenesis of subcellular organelles requires efficient delivery and concentration of subsets of proteins to specific intracellular sites for prompt assembly 1,2 . Targeting proteins to specific subcellular compartments with signal peptides or by translation of localized mRNAs is known to be involved in this process [3][4][5] . mRNA subcellular localization and local translation are evolutionary conserved and efficient mechanisms to create functional and structural asymmetries in cells by rapidly producing encoded proteins when and where required 3 . The prevalence and precise subcellular localization of certain mRNAs in association with specific organelles suggest a significant contribution of local translation 4 . Mechanisms generating asymmetric distribution of proteins in polarized cells are functionally relevant in regulating multiple aspects of cell behavior, including proliferation, migration and differentiation.
Centrioles are core structures of centrosomes involved in nucleation and formation of mitotic spindles critical for cell division 6 . Centrioles also function as basal bodies for the assembly of primary cilia and multicilia 6,7 . During multiciliogenesis, each differentiating multiciliated cell (MCC) generates hundreds of centrioles. A number of mRNAs localize and concentrate in centrioles or centrosomes 3,8,9 . Concentration of proteins and mRNAs in centrosomes might be achieved by targeting actively translating polysomes 8 . In addition, centriolar proteins are suggested to recruit the protein translation machinery that regulates the translation of specific mRNAs 10 . Still, little is known about the mechanisms of mRNA localization and local translation in centriole amplification. It is also unclear how microRNAs contribute to this process. www.nature.com/scientificreports/ Trinucleotide repeat containing 6a (TNRC6A) is a direct partner of Argonaute proteins (AGOs), which play a critical role in miRNA-induced mRNA degradation 11 . AGOs and TNRC6A have been reported in randomlydistributed cytoplasmic granules known as P-bodies, which are enriched in translation suppressors and mRNA degradation enzymes, but lack protein synthesis machinery 12 . TNRC6A and AGO2 can be also detected in other cytoplasmic ribonucleoprotein granules, such as stress granules and neuronal transport granules [13][14][15] . Cell culture studies show Tnrc6a present in most cell types, however analyses of developing and adult tissues in vivo show that levels of Tnrc6a expression can differ markedly [16][17][18] .
In an effort to gain further insights into the tissue distribution and processes associated with Tnrc6a and miRNAs, here we identifiedTnrc6a in the lung selectively expressed in MCCs of conducting airways. We found TNRC6A protein asymmetrically distributed in these cells in a not previously identified population of apical granules, which also contained AGO2. However these granules differed from P-bodies as they lacked mRNA degradation enzymes and instead were enriched or closely associated with components of the mRNA translation machinery and newly-synthesized proteins. Loss of function of Tnrc6a prevented these granules from forming and resulted in broad reduction of miRNA targets, including key components of the centriole biogenesis machinery required for multicilia formation. Our study suggests that this novel class of TNRC6A-containing apical granules can act as local translation hubs required to mediate large-scale production of centriolar proteins in MCCs. Our data also supports the idea that key components of the miRNA pathway can be utilized for mRNA recruitment and localization required for local translation.

Results
Tnrc6a is selectively expressed in multiciliated cells of the lungs. We have previously generated a Tnrc6a gt/+ reporter mouse carrying a gene trap insertion of β-galactosidase in the Tnrc6a locus to investigate expression and function of Tnrc6a in mouse development 18 . In the process of further characterizing these mutants, we performed marker analysis and X-gal staining in the adult lungs and found a striking selective pattern of β-galactosidase activity in the airway epithelium (Fig. 1a). Little to no signals were present in the alveolar epithelium and overall in the mesenchymal compartment. This pattern was similarly observed for Tnrc6a as determined by in situ hybridization (Fig. 1b). Moreover, β-galactosidase activity co-labeled with acetylated α-tubulin (Ac-α-tub), indicating that Tnrc6a was selectively expressed in MCCs of the airway epithelium (Fig. 1c).
To further confirm expression and cell type specificity, we performed immunofluorescence (IF) staining of lung sections with the human index serum (termed as 18,033), widely used for detection of TNRC6A protein 19,20 . This showed cytoplasmic signals restricted to Ac-α-tub positive cells, further corroborating TNRC6A selectivity to MCCs (Fig. 1d). To investigate when these granules are established and their association to multiciliogenesis, we analyzed airway epithelial progenitors (basal cells) from adult mouse trachea undergoing differentiation in air-liquid interface (ALI) cultures 21 . Analysis of TNRC6A (18,033) and FOXJ1, an early marker of MCC cell fate 21 showed TNRC6A signals first detected at ALI day2 in emerging FOXJ1+ cells. By day 3-4, as these cells underwent large-scale centriole amplification to initiate multiciliogenesis 21 , TNRC6A+ cytoplasmic granules became abundant in MCCs (Fig. 1e). Signals were subsequently diminished in more mature MCCs, which by then expressed strong Ac-α-tub (Fig. 1f).
TNRC6A is enriched in a novel class of apical epithelial granules that lack mRNA degradation enzymes. To gain additional insights into the subcellular distribution of these granules in MCCs, we analyzed a series of Z-stack images of ALI day 4 cultures double-stained with the human index serum and FOXJ1. This revealed an overall population of granules heterogeneous in size, randomly distributed throughout the cytoplasm of MCCs but also identified a population of apically-localized granules of smaller size in these cells. Immunostaining using a monoclonal TNRC6A antibody (4B6) and acetylated alpha tubulin showed these apical granules underneath the multicilia of differentiating MCCs (Fig. 2a,b). Their localization in MCCs was further confirmed in ALI cultures transduced with a lentiviral vector expressing a TNRC6A-EGFP fusion protein 21 (Fig. 2c).
The human index serum is also known to label P-bodies (processing bodies), cytoplasmic ribonucleoprotein (RNP) granules, which contain TNRC6A and various RNA pathway enzymes involved in decapping and degradation of mRNAs 22 . To further identify potential differences in the populations of MCC granules, we stained ALI cultures with human index serum and antibodies against components of the P-bodies. IF for 18,033 doublelabeled with EDC4, DCP1A, DDX6 and XRN1 showed these signals highly expressed in the randomly-localized population of cytoplasmic granules (P-bodies), but largely undetectable in the apical granules that we also identified using the anti-TNRC6A (4B6) antibody in the same MCC (Fig. 2b,d,e, Supplementary Fig. 1). TNRC6A is a known partner of AGO2 12 . Co-IF staining of human index serum with AGO2 showed co-localization of AGO2 in both classes of granules in MCC (Fig. 3a). Notably, lentiviral gene transduction of AGO2-EGFP fusion protein showed the strong EGFP signals enriched apically underneath the cilia of differentiating MCCs (Fig. 3b).
This supported the idea that TNRC6A-containing apical granules represented a potentially novel class of granules distinct from P-bodies in size, subcellular distribution and absence of enzymes needed for mRNA degradation.
microRNAs are crucial components of the TNRC6A apical granules during multiciliogenesis. We examined the possibility that microRNAs previously reported in MCCs could be locally enriched where the TNRC6A apical granules were abundant. Indeed, ISH for miR-449a, miR-34a, miR-34b, representative MCC miRNAs [23][24][25][26] , showed strong signals apically, adjacent to the base of multicilia (Fig. 3c, Supplementary www.nature.com/scientificreports/ Fig. 2). Thus, the data suggested that the TNRC6A apical granules contained AGO2, miRNAs and presumably their targets. We then asked whether microRNAs were necessary for formation of these apical granules during MCC differentiation. Expression of the miR-34/449 family is required for basal body maturation and apical docking during multiciliogenesis 23 . We examined ALI airway epithelial cultures from miR-34/449 null mice in which all three loci encoding miR-34/449 were deleted 24 . Analysis of ALI day 4 cultures in WT differentiating MCCs showed the typical abundant TNRC6A apical granules revealed by human index serum IF (18033 single-labeled Fig. 3d, left) and the larger randomly distributed double-labeled P-bodies (18033-DCP1A-labeled granules). By contrast, in miR-34/449 null cultures, apical TNRC6A granules were notably decreased, absent or disorganized in contrast to the P-bodies, which were largely preserved (Fig. 3d, right). Given the high abundance of miR-34/449 in MCCs (~ 70% of all miRNAs) 23 , the data strongly suggest that miR-34/449 is a key cellular component required  TNRC6A-containing apical granules are sites of newly-synthesized proteins in differentiating MCCs. Our study identified TNRC6A, AGO2 and miRNAs in apical granules lacking enzymes for mRNA decay in differentiating MCCs. We asked whether TNRC6A was involved in mRNA localization and local translation at the apical domain in these cells, which is a site of active multicilia formation. Puromycin has been reported to label newly-synthesized proteins, since it mimics aminoacyl tRNA and can enter the ribosome A site to terminate translation by ribosome-catalyzed covalent incorporation into the nascent polypeptide www.nature.com/scientificreports/ C-terminus 27 . O-Propargyl-puromycin (OPP), an alkyne analog of puromycin, has been used to detect localized translation [27][28][29] .
To visualize newly-synthesized polypeptides in differentiating MCCs, ALI Day 4 mouse airway epithelial cultures were labeled with OPP for 30 min before termination and subsequently analyzed by IF-confocal microscopy. OPP staining revealed strong signals localized in apical foci underneath emerging cilia (Fig. 4a). The OPP foci were cell type-specific and dynamic, as they were not detected in mature MCCs or in other airway epithelial cell types. Notably, OPP signals were abolished by pretreatment with cycloheximide ( Fig. 4b), supporting the specificity of this approach. OPP foci were similarly found in differentiating MCCs from human ALI airway epithelial cultures, suggesting conservation of this mechanism in mammalians ( Fig. 4c). Importantly, OPP signals were undetected in the randomly-localized P-bodies. Co-IF of OPP with the human index serum (18033) and DCP1A in ALI day4 cultures showed OPP-18033 signals overlapping apically but no 18033 overlapped with the DCP1A positive P-bodies within the same MCC (Fig. 4d).
A closer analysis of the pattern of OPP staining showed that the OPP foci varied considerably in size and signal intensity In both human and mouse airways even within the same MCC, suggestive of a dynamic regulation (Fig. 4e). Morphometric analysis revealed OPP foci ranging from 300 to 2000 nm in diameter and signal intensity ~ 2.7 folds higher locally compared to the neighboring area (OPP mean optical density, P < 0.0001). Overall OPP staining could be identified in small and large foci in the apical region of differentiating MCCs. The small OPP foci overlapped extensively and at various degrees with the small TNRC6A-containing granules (labeled with 18033). By contrast the large OPP foci also stained apical granules but these were not 18033 (TNRC6A) positive or P-bodies (Fig. 4f,g and diagrams).
Newly-synthesized proteins and translation machinery are found in apical granules during centriole amplification. The distinct association of TNRC6A with OPP foci led us to further investigate what these sites of newly-synthesized protein represented in differentiating MCCs. Their vicinity to apicallylocated ciliary structures suggested that OPP could be labeling sites of local translation of centriolar components required for multicilia formation 30,31 . To examine this possibility we performed co-immunostaining of OPP with key centriole biogenesis markers in ALI day 3-4 cultures. This revealed the smaller OPP foci (~ 400-800 nm in diameter), abutting centrioles (300-400 nm in diameter), as identified by CENTRIN3, and colocalized with DEUP1, a marker of deuterosomes 30,31 (Fig. 5a,b).
The spatial relationship of TNRC6A, CCP110, DEUP1, and TNRC6A with the small OPP foci was examined in differentiating MCCs by double and triple-IF stainings ( Fig. 5a-d). The CCP110 signals were found surrounding double positive DEUP1/OPP small foci, suggesting the co-localization of TNRC6A positive granules with procentrioles surrounding deuterosome. Interestingly, this co-IF analysis showed no overlap of the large OPP foci with CENTRIN3, CCP110, DEUP1, or TNRC6A. Instead, the large OPP foci (~ 800-2000 nm in diameter) colocalized with PCM1 (Fig. 5e), a component of the pericentriolar material widely reported in fibrous granules 30,31 .
We then asked whether components of the translation machinery were present at the sites of OPP expression. Indeed, IF analysis of ALI cultures showed the small OPP foci strongly co-labeled with eIF3B and phosphorylated RPS6 (p-RPS6) (Fig. 6a,b), indicative of active translation in the apical granules we had previously shown to contain TNRC6A (Fig. 5). Single-molecule fluorescence in situ hybridization (smFISH) revealed ribosomal protein 3 Rpl3 mRNA (a target of mir34/449) largely expressed in the OPP small foci of TNRC6A-granules (Fig. 7a). The Rpl3 distribution differed markedly from that of MLS mRNA (RNA transcribed from the mitochondrial L-strand DNA). Although known to be abundantly expressed in differentiating MCCs, MLS was neither co-localized nor associated with any OPP in these cultures (Fig. 7b). Interestingly, in the large OPP foci (associated with fibrous granules, Fig. 5) we detected expression of p-RPS, eIF3B and ribosomal RNAs (18S, 28S) surrounding the OPP signals (Figs. 6c,d, 7c,d). A similar subcellular arrangement has been described in the sponge bodies, cytoplasmic organelles of the developing Drosophila oocyte, in which the translational activator Orb and ribosomes are located at the edge or surrounding these bodies to promote local translation 32 . The lack of PCM1-18033 co-labeling in the large OPP foci suggested that, in fibrous granules, newly-synthesized protein is regulated by a TNRC6A-independent mechanism. Altogether the data strongly suggested that TNRC6A-contaning apical small granules are active sites of local translation during centriole amplification in differentiating MCCs.
Disruption of Tnrc6a expression leads to reduced CCP110, abnormal centriologenesis and defective multicilia formation. Next, we investigated whether Tnrc6a expression was required for the formation of the apical granules in differentiating MCCs. For this, we examined ALI airway epithelial cultures from a Tnrc6a null mice carrying an EGFP reporter to identify cells committed to MCC fate (Tnrc6a null ; FOXJ1-EGFP, see "Methods"). IF staining of ALI day 4 with human index serum showed that disruption of Tnrc6a specifically abolished the apically-localized granules, but not the cytoplasmic P-bodies in FOXJ1-EGFP-labeled MCCs (Fig. 8a). We asked how this could affect production of CCP110, a key centriolar protein involved in ciliogenesis and known target of miR-34/449 24 . Thus, we co-cultured airway progenitors from wild type and Tnrc6a null ; FOXJ1-EGFP mutants and differentiated these cells in ALI cultures. This allowed to compare potential differences in the intensity of signals between Tnrc6a-deficient and WT cells side by side, using the latter as internal controls. IF analysis of CCP110 showed strong signals in WT cells but expression was drastically reduced in the Tnrc6a null ; FOXJ1-EGFP MCCs (Fig. 8b). The presence of FOXJ1-EGFP in Tnrc6a mutant cells, suggested no significant defect in MCC fate determination (Fig. 8a,b). This was further supported by analysis of E18.5 lungs showing that the proportion of FOXJ1 positive cells in airways from WT and Tnrc6a null mutants was essentially the same ( Supplementary Fig. 4).
IF staining of ALI co-cultures of Tnrc6a null ; FOXJ1-EGFP and WT epithelial cells revealed marked differences in levels and distribution of proteins involved in centriole amplification. For example, in Tnrc6a null cells  www.nature.com/scientificreports/ signals for CEP135 and PCM1 were clustered in a corner of the differentiating MCC, characteristic of an early stage of centriole amplification (Fig. 9a,b) 33,34 . Moreover, in mutant MCCs Centrin3 staining was drastically reduced (Fig. 9c). These changes were compatible with a significant defect of Tnrc6a mutant cells in generating the large number of normal centrioles/basal bodies required for multiciliogenesis. Consistent with a defect in centriologenesis, few cilia formed in differentiating MCCs of Tnrc6a mutants when co-cultured with WT cells (Supplementary Fig. 5a). The defect in multicilia formation was further confirmed by examining the trachea of Tnrc6a null mutants at E18.5 ( Supplementary Fig. 5b-d). Efficient disruption of Tnrc6a was demonstrated by IF staining with the monoclonal antibody (4B6), which showed TNRC6A positive granules in tracheal MCCs of wild type but not of mutants ( Supplementary Fig. 5b). Co-staining with Ac-α-tub and TNRC6A demonstrated a significantly reduced multicilia staining in Tnrc6a null tracheas. Scanning electron microscopy of tracheae from mutants showed the majority of MCCs (83%) with shorter and sparser cilia as compared to WT ( Supplementary  Fig. 5c,d). The cilia truncation was consistent with an immature MCC phenotype. We found no difference in FOXJ1 staining between WT and Tnrc6a null lungs ( Supplementary Fig. 4), suggesting no defect in MCC fate determination. Transcriptome analysis of WT and Tnrc6a null lungs did not detect significant changes in expression of genes associated with proliferation or differentiation of other cell types of the lung. Moreover, no gross abnormalities were observed in these embryos or in the overall morphology of their lungs and no compensatory Together, these data strongly support that idea that loss of Tnrc6a leads to a defect selectively in MCCs, the same cell type where we found Tnrc6a highly expressed in WT lungs.
Disruption of Tnrc6a expression leads to reduction, rather than stabilization of miRNA targets in mutant lungs. TNRC6A has been traditionally described as a component of the RNA-induced silencing complex in microRNA (miRNA)-mediated gene suppression, repressing translation or promoting mRNA degradation. This is consistent with previous findings of defective miRNA function with increased miRNA target expression in yolk sac endoderm of Tnrc6a null mice 18 . We examined the subcellular localization of TNRC6A in E9.5 yolk sacs and found endodermal signals colocalized with AGO2, EDC4 and other mRNA degradation enzymes in randomly-distributed cytoplasmic granules suggestive of P-bodies 17 (Supplementary Fig. 7). Intriguingly, our analysis of TNRC6A expression in the lung showed evidence of its localization in differentiating MCCs in apical granules distinct from P-bodies in size, subcellular distribution, absence of enzymes needed for mRNA degradation and enrichment in translation machinery components. Moreover we had evidence of TNRC6A apical granules as sites of active translation crucial for large-scale centriole amplification in multiciliogenesis. This suggested that TNRC6A could have a distinct role in the lung, compared to the yolk sac, where it promotes www.nature.com/scientificreports/ miRNA target degradation. To investigate this possibility we examined the behavior of miRNA in the lung and yolk sac from control and Tnrc6a null mice. We performed gene set enrichment analysis (GSEA) using a dataset containing the most evolutionary conserved targets of 83 conserved miRNAs as determined by Targetscan (http:// www. targe tscan. org). GSEA of these genes in yolk sac (GSE30244) showed targets of most miRNAs (78 out of 83) significantly enriched in the list of genes upregulated in Tnrc6a null yolk sacs (Supplementary Fig. 8). This was consistent with a role for Tnrc6a in the destabilization of miRNA targets 18,35 . Strikingly, similar analysis using the Tnrc6a null lung dataset (GSE89327) showed that targets of most miRNAs (79 out of 83) were enriched among the genes significantly reduced in Tnrc6a mutant lungs, but not among the genes upregulated ( Supplementary Fig. 9). This was consistent with the idea that, in the lung, Tnrc6a stabilizes miRNA targets instead of promoting degradation.
Indeed, analysis of miR-34/449 predicted targets in both yolk sac and lung datasets showed that, among 94 transcripts with conserved binding sites significantly altered in Tnrc6a null lungs, the majority (80%) was downregulated. By contrast 75% of the conserved miR-34/449 targets altered in yolk sac mutants were upregulated (72 total). Among the top 30 most downregulated genes in mutant lungs, several have been implicated in multicilia formation and 30% of these genes are predicted targets of miR-34/449, key in multiciliogenesis 23,24 (Supplementary Table 1, Supplementary Fig. 10). For example, Spef2 and Rsph1 were strongly expressed in MCCs of the developing lung and their signals were reduced in Tnrc6a null mutants. qRT-PCR further confirmed   Fig. 10). As demonstrated in other systems, genetic deletion of Tnrc6a disrupted miRNA function without affecting miRNA expression or stability 18 . Analysis of miR-449 (from the miR34-449 cluster) and miR-146 showed no difference in expression between WT and Tnrc6a null mutants ( Supplementary Fig. 11). Altogether, our data strongly suggested that, in the lung, Tnrc6a was not involved in degradation of miRNA targets but rather appear to be required for mRNA stabilization, and likely promoting protein synthesis.

Discussion
Here we identified TNRC6A selectively expressed in MCCs of lungs in vivo, concentrated in a novel class of apical granules containing AGO2, miRNAs and their targets, adjacent to immature centrioles. Although present in adult airways, these granules are mostly abundant in differentiating immature MCCs. Remarkably, components required for mRNA degradation are missing in these granules, while present in P-bodies within the same MCCs. Instead, they are enriched in components of the mRNA translation machinery and show evidence of concentrated newly-synthesized proteins. We found that, in the lung, loss of Tnrc6a leads to broad reduction instead of accumulation of miRNA targets, suggesting a role in target mRNA stabilization, rather than in degradation. Proximity-dependent biotinylation assays using centriolar and cilia proteins as baits have reported CCP110 in close proximity to TNRC6A 36 . Here we show CCP110 and other centriolar proteins localized to the apical TNRC6A granules and no evidence of their association with enzymes involved in mRNA degradation. Notably, loss of Tnrc6a resulted in a drastic reduction of expression of centriolar proteins, abnormal centriologenesis and, ultimately, defective MCCs.
Cytoplasmic RNP granules in mammalian cells, including P-body, stress granule, neuronal transport granule and germ granule are generally linked to translation repression. mRNAs are not translated in stress granules where translation initiation factors are concentrated. Indeed, we found no evidence of newly-synthesized polypeptides in P-bodies in differentiating MCCs. Instead, OPP and translational machinery components were present in the apical TNRC6A-containing granules. AGO2 mainly functions as a miRNA binding protein to trigger miRNA-mediated mRNA degradation 37,38 . Interestingly, AGO2 was originally purified and cloned as a translation initiation factor and named EIF2C2 39 . It is known that AGO2 can interact with EIF1A in the biogenesis of miRNAs 40 . miiRNAs are well known as a negative regulators of gene expression. However, miRNAs have been also shown to stimulate mitochondrial translation to activate translation of miRNA targets in quiescent cells and to enhance translation initiation mediated by HCV-like IRESes [41][42][43] . There is evidence that AGO2 (a direct TNRC6A-binding partner) associates with ribosomes, functioning as a translation activator to mediate translation upregulation 44 . Recruitment of TNRC6A to AGO2/miRNA complex typically triggers a cascade of events for translation suppression and target mRNA degradation; however evidence of TNRC6A interaction with poly (A) binding proteins (PABP) also suggests a role in translation [45][46][47][48] . Our findings, together with reports of active translation associated with FUS granules 49 , point to a more prevalent role of cytoplasmic RNP granules in active local translation than previously suspected.
Recent studies have identified TNRC6A or AGO2 adjacent to the basal body of primary cilia or centrosomes 50,51 , suggesting that the spatial relationship between TNRC6A/AGO2 positive granules and centrioles is conserved in both primary cilia and multicilia (as we report here). We found newly-synthesized proteins (OPP) also concentrated in granules at the base of primary cilia (Fig. 10). However, primary cilia remained intact in cultured Tnrc6a null cells in contrast to our findings in multicilia.
The precise mechanism by which TNRC6A promotes mRNA translation is under investigation. Based on our current observations, we propose that induction of Tnrc6a in differentiating MCCs and their accumulation in the apical granules with AGO2 and miRNAs, such as miR-34/449, lead to recruitment of target mRNAs key for multiciliogenesis, as exemplified here by those encoding centriolar proteins. The lack of mRNA degradation enzymes in the TNRC6A granules allows stabilization of these mRNA targets for local translation. Indeed, TNRC6A is known to directly bind to poly(A) binding proteins (PABP) 47,48 , which recruit elongation factors to initiate protein synthesis 52,53 . Moreover, AGO2, a binding partner of TNRC6A, directly interacts with eIF1A 40 and presumably other elongation factors recruiting them to these apical granules. The presence of OPP and elongation factors in the TNRC6A-containing apical granules support the idea that these are site of local newlysynthesized proteins. The key role for Tnrc6a in this process was seen by the selective loss of the apical OPPlabeled foci when these granules were abolished in Tnrc6a-deficient cells (while OPP was maintained elsewhere in these cells.
In summary, our findings suggest that components of the miRNA pathways can be organized into subcellular hubs for target mRNA localization, stabilization and local translation crucial for differentiation of airway MCCs. These observations further provide strong evidence of the critical role of mRNA localization for efficient delivery and concentration of subsets of proteins to specific intracellular sites for prompt organelle biogenesis during cellular differentiation.

Material and methods
Mouse breeding, genotyping and monitoring. Tnrc6a gt/+ reporter mice carrying a gene trap insertion of the β-galactosidase gene in the Tnrc6a locus and Tnrc6a gt/gt mutants were generated on a mixed genetic background as described 18 . Tnrc6a mutants were backcrossed to C57BL/6 for ten generations to overcome embryonic lethality. Mice are housed in a non-barrier animal facility at Columbia University Medical Center. The genotyping is performed as reported 18 . miR-34/449 null mice were bred and genotyped as described 23 . Foxj1/ EGFP/Tnrc6A mice were generated by breeding Tnrc6A gt/+ mice to Foxj1-EGFP mice (Jax, B6; C3-Tg (Foxj1-EGFP)85Leo/J) 54   Histological analyses. Tissues were dissected and fixed overnight in in 4% paraformaldehyde (Ted Pella, #18505) overnight at 4 °C, then processed either for frozen section or paraffin section by following standard procedures. All the blocks were sectioned at 10 μm either for immunostaining or for in situ hybridization.
β-Galactosidase staining. This was performed according to protocol described previously 18 . Briefly, adult lungs of Tnrc6a gt/+ mouse were perfused intratracheally with 0.25% glutaraldehyde (Electron Microscopy Science, #16220) at a constant pressure of 30 cm H 2 O for 30 min, then perfused with 1XPBS for 2 h, followed with staining buffer (5 μm C6N6FeK3, 5 μm C6N6FeK4, 2 mM MgCl2, 1 mg/ml X-gal in 1XPBS) for 30 min. Then the trachea was tied and the whole lungs were incubated in the staining buffer in a 50 ml tube overnight at 37 °C. After washing in 1XPBS three times (2 h for each wash), lungs were fixed again using 4% PFA (Ted Pella, #18505) for 2 h and then processed for embedding following standard procedures. Sections of LacZ-stained lungs were either subjected to H&E staining or co-stained with antibodies specific for MCCs.  Single-molecule fluorescence in situ hybridization (smFISH). mTEC epithelial cultures were fixed for 15 min in 4% PFA, followed by three washed in PBS then transfer to 70% ethanol for storage. In situ hybridization and imaging were performed as previously reported 28 . Fluorescence labeled DNA oligo probes are from Itzkovitz's Laboratory. All probes are labeled with Cy5 except rRNA 18s (TMR).

Immunofluorescence (IF) and immunohistochemistry (IHC
O-Propargyl-puromycin (OPP) labeling and imaging. mTEC cultures were treated with either 50 μg/ ml Cycloheximide (Sigma-Aldrich, C7698) or DMSO control for 15 min, followed by 30 min incubation with 50 μM OPP (Jena Bioscience, NU-913-5). Cultures were then washed with room temperature PBS and fixed with 4% of PFA, followed by three washes in PBS. OPP features an alkyne group that can be detected and imaged by the addition of azide-coupled fluorophores in the highly specific click reaction 27,28 . The click reaction was performed subsequently to the IF staining by making use of the Click-iT Alexa Fluor 488 or Click-iT Alexa Fluor 647 Imaging Kits (Thermo Fisher Scientific, C10337 and C10340). The kits were used according to the manufacturer's instruction with the exception of an additional 20-fold dilution of the working solution of the Alexa Fluor azide dye in DMSO.

Immunofluorescence (IF) signal quantitation and statistical analysis.
To quantify the IF staining of newly synthesized proteins (OPP) and other centriolar proteins, Z-stack of confocal images were taken for the entire cells. For OPP staining, several planes covering the entire OPP foci were chosen and the maximal projection images of these planes were acquired. Then the areas encompassing the OPP foci and background were marked using ImageJ and the mean optical intensity was calculated. For centriolar proteins, we first generated the maximal projection image for entire cells, and drew the cell boundaries. Then the mean optical intensities were measured by ImageJ. Data were presented as the mean ± SEM and subjected to 2-tailed, paired Student's t test. P < 0.05 was considered significant. In all experiments a minimum of 3-4 replicates (ALI cultures or lung sections) were used for analyses.

Data availability
All procedures and experiments were performed in accordance with the relevant guidelines, regulations and protocols approved by CUIMC (Institutional Animal Care and Use Committee approval: WVC IACUC #: AC-AABF2567). All methods are reported in accordance with ARRIVE guidelines. Data referenced in this study from transcriptomics of control and Tnrc6a null mice in lung (E18.5) and yolk sac (E9.5) are available in the National Center for Biotechnology Information Gene Expression Omnibus (GEO) and are accessible through the GEO Series accession number GSE89327, GSE30244 and GSE89327. The data is available in link https:// drive. google. com/ drive/ folde rs/ 15JZ_G_ kxdZS pjS9d w1tIH fZEds-dUNwk? usp= sharin. All other relevant data are available from the corresponding author upon request.